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78 GS550 air leak maybe?

  • Thread starter Thread starter Jmcb
  • Start date Start date
J

Jmcb

Guest
I have a 78 GS550. Over the winter I did a dyna electronic ignition.. And shimmed the valves. New plugs etc.. Has been running pretty nicely all spring. About a week ago I notice a couple of random backfires every now and again. Usually off the line. Just when I hit the throttle.. Pulled the plugs today.. Every plug looks a bit black and and souty.. Except for cylinder 4. Which had a white electrode and ceramic. So air leak on 4?? Where would be the most likely place for an air leak on a individual cylinder?? Carb boot?
 
I don't like souty plugs.. What you got for airfilter box,stock setup? Is filter element clean?
 
Yes.. Stock air box and filter. The filter was new last summer. Maybe pull it and give it a clean. But perplexed about the 1 lean looking one? What could cause 1 cylinder to run lean? Assuming all 3 carbs are set up the same?
 
Never assume anything on a 40 year old bike
Pull the carbs and check everything
 
One prime suspect would be the o-ring where the intake boot meets the cylinder head.

Another prime suspect would be a broken tip on a pilot fuel screw, but that would be rather noticeable, as it would appear to idle on just three cylinders, then the fourth one would kick in about 1/4 throttle.

.
 
I'd drop #4 bowl and hope that something might have got sucked up into pilot jet area and can be extracted - you might get lucky!
 
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